Attention is drawn on the U.S. Pat. No. 6,141,086 as well as to                G. W. Kammermann, Selected Papers on Laser Radar, SPIE Optical Engineering Press, 1997        J. W. Goodman, Comparative performance of Optical-Radar Detection Techniques, IEEE Transactions on Aerospace and Electronic Systems, Vol. AES-2(5), 526ff., 1966        CO2 LADAR modulation trade of studies, coherent Infrared Radar Systems and Applications II, Proc. SPIE, Vol. 415, 155ff. (1983)        WO 98/30881        GB-A-2401738        S. Nissilä et al., A Fibre Laser as a Pulse Source for Laser Rangefinder system, SPIE, Vo. 1821 (1992), 375.        
To resolve the above object there is proposed a laser-system in master-oscillator/power-amplifier (MOPA) configuration which comprises a pulsed diode master oscillator and a pumped active optical fibre (fiber) power amplifier downstream said master oscillator and wherein substantially all guides of laser light are optical fibres.
In one embodiment of the laser system there is provided a detector unit for incoming pulsed laser light which detector unit is operationally connected to an evaluation unit. The evaluation unit performs evaluation on multiple incoming laser pulses. By multiple pulse evaluation the system detection accuracy is improved in spite of limited pulse energy of the emitted laser light.
In a further embodiment the active optical fibre power amplifier is pumped by a pumping diode.
Still in a further embodiment of the laser system the active fibre power amplifier is gain modulated. This allows e.g. to cope with time-varying intensity of the emitted laser pulses and/or to improve signal-to-noise ratio by specifically controlled gain modulation.
In one embodiment of the laser system the active fibre power amplifier is gain modulated by at least one of intensity variation of pumping light, spectrum variation of pumping light, pumping pulse width variation, length of active fibre, spectral shift of an optical filter characteristic.
In a further embodiment of the system according to the present invention the active fibre power amplifier is pumped in a pulsed mode and pulsating pumping is synchronized with pulsed operation of the diode master oscillator.
Still in a further embodiment of the system according to the present invention the active fibre power amplifier is an adjusting unit within a negative feedback loop whereat a physical entity of the laser beam downstream the active fibre power amplifier is sensed as a measured value to be controlled and is compared with a desired value. The gain of the active fibre power amplifier is adjusted in dependency of the result of the addressed comparing.
In one embodiment the addressed physical entity sensed is one of signal-to-noise ratio in the laser beam and of intensity of said laser beam.
In a further embodiment the system according to the present invention comprises a pass-band optical fibre filter downstream the active fibre power amplifier. In a further embodiment the addressed pass-band optical fibre filter has a filter characteristic the spectral position of which being controllably shiftable.
In a further embodiment the just addressed filter characteristic is shiftable in dependency of a temperature.
Still in a further embodiment the just addressed temperature is dependent from temperature of the diode master oscillator.
By controlling the spectral position of the addressed filter characteristic in dependency of the temperature of the diode master oscillator and taking into account that the spectral band of laser light emitted from the diode master oscillator varies with respect to spectral location and in dependency of the temperature at this oscillator, it becomes possible to realize a laser beam downstream the addressed pass-band optical fibre filter which is substantially unaffected by temperature caused spectral shift of the laser light spectral band.
In a further embodiment the system comprises a stabilizing optical fibre filter within the diode master oscillator being decisive for the spectrum of laser light output from the diode master oscillator. The system comprises a pass-band fibre optical filter downstream the active fibre power amplifier. The spectral locations of the filter characteristics of the stabilizing and of the downstream pass-band optical fibre filter are matched.
Due to the fact the instantaneous spectral location of the filter characteristic of the stabilizing and of the downstream filter are matched and thus substantially equally shifting e.g. due to variations of temperature, the output spectral band of laser light from the diode master oscillator shifts spectrally equally as the filter characteristic of the downstream pass-band filter. Thereby, again, substantially constant output intensity of the laser beam is achieved in spite of spectral shift of the output spectral band of laser light. Matching is simplified if the temperature sensed at one common location controls the respective spectral shifts of the two filters, or these filters are thermally narrowly coupled.
In a further embodiment of the system according to the present invention such system comprises more than one of the addressed active fibre power amplifiers.
Still in a further embodiment the system comprises an output/input coupler unit with an input fibre which is operationally connected to the output of the active fibre power amplifier and with an output fibre which is operationally connected to the input of a detector unit. An output/input fibre as provided is operationally connected to a laser light emitting and receiving optic.
Thereby still in fibre technology a one input/output laser system is realized.
In one embodiment the just addressed coupler unit comprises an optical circulator.
Still in a further embodiment, the system comprises an optical fibre, one end thereof being operationally coupled to a transmitter optic for laser light dependent from laser light output from the master-oscillator/power-amplifier, the other end of the fibre being operationally connected to an output of the pumped active fibre power-amplifier. Thereby the one end of the fibre is conceived for substantially determining the divergence of the laser beam output from the transmitter optic. Thereby additional lenses are omitted, making the system less expensive, more robust and more compact.
The addressed transmitter optic is thereby, in one embodiment, also a receiver optic of the system.
In a further embodiment, the addressed optical fibre is a active fibre.
One device according to the present invention with the addressed laser system is portable or even handheld. In one embodiment the device incorporating the addressed laser system is a range finder or target designator unit operative for ranges up to 1 km and even up to at least 10 km.
In one embodiment the device is built in a tank or submarine or in a portable observation instrument.
Attention is drawn on the fact that the content of the European application No. 05 000 669.1 dated Jan. 14, 2005 as well as the content of the European application No. 04 029 867.1 dated Dec. 16, 2004 upon which the present application resides with respect to priority, is considered as a part integrated by reference to the present disclosure.
The inventions under all their aspects and combinations shall now be exemplified by means of the following figures.